Typical bipolar fabrication techniques involve forming a base region by directing an implant species into an already-formed collector region. In a typical process, the surface of the silicon substrate is covered with approximately 800-1400 .ANG. of oxide. The thickness of the oxide is chosen to prevent subsequent ion channeling and provide emitter-base isolation. The base implant energy and dose are chosen in such a way as to insure that the peak base dose occurs at the oxide/silicon interface and that the total base dose yields a Gummel number between 1.times.10.sup.12 and 4.times.10.sup.12 atoms/cm.sup.2.
However, in a production environment, it is difficult to precisely control the thickness of the oxide. Often, there may be a variation within or between wafers or from lot to lot of as much as 10% in oxide thickness. Since the oxide thickness essentially governs the base dose and profile, there may be a substantial variation in the total amount of impurity doping in the base and the base width. Thus, there may be a substantial variation in the Gummel number of the base and the base profile and width, and substantial variation in transistor performance.
For many bipolar transistors, especially those with polysilicon emitters, thick oxide is required over the base to isolate the base from the external circuit or from capacitive effects of the polysilicon emitter. The comparatively thick (800-1400 .ANG.) oxide is grown before implantation because growth after implantation would cause undue base diffusion which increases the base width and therefore causes a significant degradation in transistor performance.